The special theory of relativity (published in 1905) describes the behavior of objects in motion (at constant velocity) in terms of their frames of reference, essentially refining the classical mechanics (physics of bodies in motion) delevoped by Galileo and Isaac Newton, among others. After being reformulated in terms of a proposed four-dimensional "spacetime" by Hermann Minkowski in 1908, the theory became a powerful conceptual and computational tool.

The general theory of relativity (completed in 1915) explains gravitation in terms of the curvature of spacetime. It extends the results of special relativity to non-intertial (that is, accelerating) frames of reference.

It must be noted that Einstein's relativity has nothing to do with philosophical relativism in any form (moral, ethical, cultural, etc.); they merely sound similar and ultimately derive from the same Latin root. The name "relativity" comes from the fact that both theories are based on more general concepts called principles of relativity — essentially that the laws of physics should not depend on special frames of reference.